The present invention relates to an improved edge dam for use with applicators of the trailing blade type for applying a coating liquid to a moving web of paper.
Conventional applicators of the trailing blade type include means for applying coating liquid to a paper web that is usually supported and carried by a backing roll. Such applicators include a chamber having an opening extending across and parallel to the web, together with a doctor blade at a trailing side of the opening for leveling the coating and a front wall or orifice plate extending from a leading side of the opening toward the web and defining an orifice for exit of excess coating material from an application zone between the wall and doctor blade. Coating liquid is supplied to the chamber and thence through the chamber opening into the application zone, and to seal end spaces between the front wall and doctor blade at side ends of the zone to prevent escape of coating material laterally of the web, edge dams are provided thereat.
For applicators of the foregoing type, the edge dam often comprises a felt, plastic or metal material configured to fill a somewhat triangular shaped opening at either end of the application zone defined by the doctor blade, orifice plate and backing roll. Ideally, the edge dam absolutely prevents leakage of any coating liquid to exterior of the applicator. However, that can require that the edge dam be in contact with the paper web, and wear or abrasion of the edge dam or marking and tearing of the web may occur. Consequently, the edge dam is only positioned closely adjacent to, but spaced from, the web, and some coating liquid seeps therepast.
In use of edge dams, an effort is made to minimize leakage of coating liquid. However, it is usually very difficult, if not impossible, to mount edge dams at the side edges of the application zone in sufficiently close proximity to the paper web to allow only an acceptably small amount of leakage. In practice, coating liquid often leaks past the edge dam, with the result that the web edges become contaminated by stickers of coating liquid, which bind together adjacent edges of a wound paper roll and prevent the roll from later being properly unwound.
If leakage of coating liquid past edge dams could be eliminated or at least significantly minimized, it would then be possible to coat only the center portion of a paper web, leaving narrow uncoated strips along opposite side edges of the web. This would advantageously reduce or eliminate coating material buildup on backing rolls following the coater as well as edge stickers on the coater backing roll and web. Unfortunately, conventional edge dams do not ordinarily permit the foregoing to be accomplished.